HP-UX TCP/IP Performance White Paper, March 2008

A window size of 32768 bytes is enough to allow 10 Mbit/s of throughput out to a RTT of roughly 25

milliseconds. It would allow 100 Mbit/s of throughput out to roughly 2.5 milliseconds, and 1000 Mbit/s of

throughput out to roughly 0.25 milliseconds.

Typically, the round-trip time on a local LAN is <= 1 millisecond. For a terrestrial (no satellites) link across

the continental US the RTT is anywhere between 30 and 100 milliseconds, though it can be higher. If there

is a satellite hop involved, the RTT will be a minimum of roughly 250 milliseconds for each satellite hop,

and if a satellite hop is used in both directions, that means 500 milliseconds. For a WAN the RTT is

(generally) more a function of distance and the speed of light than it is a function of bit-rate - the RTT for a

satellite 1.5 Mbit/s link and a 45 Mbit/s link would be about the same.

tcp_recv_hiwater_lfp:

The maximum size for the receive window for "long, fat pipe"

interfaces such as Fibre Channel which provide high bandwidth

with high latency. [4096,-] Default: 65536 bytes

tcp_recv_hiwater_lnp:

The maximum size for the receive window for "long, narrow pipe"

interfaces such as PPP over a 56 kb modem which provide low

bandwidth with high latency. [4096,-] Default: 8192 bytes

tcp_rexmit_interval_initial:

Initial value for round trip time-out, from which the retransmit

time-out is computed. [1,20000] Default: 3000ms

When TCP first sends a segment to the remote, it has no history about the round-trip times. So, it will use

tcp_rexmit_interval_initial as a best guess for its first retransmit timeout setting. Setting this value

too small will result in spurious retransmissions and could result in poor initial connection performance. If

this value is set too large, TCP may not be able to send many retransmissions before it reaches the

tcp_ip_abort_interval or tcp_ip_abort_cinterval and could result in a spurious connection

abort.

tcp_rexmit_interval_initial_lnp:

Same as tcp_rexmit_interval_initial, but used for devices with

the LNP (Long Narrow Pipe) flag set. [1,20000] Default: 3000ms

tcp_rexmit_interval_max:

Upper limit for computed round trip time-out. [1,7200000]

Default: 60000 (1 minute)

It should almost never be the case that this is in need of changing. If you make the value larger, there will

be fewer total retransmissions. Unless you also increase tcp_ip_abort_interval, this could lead to an

increase in "false positives" on connection failures.

Setting this value too low could also contribute to what is called "congestive collapse" of the network -

where the network is ~100% occupied with retransmissions instead of new data. It would do this by